Add ATTR_REL_POS to NIMBUS system xml

ATTR_EFF_HYBRID and ATTR_EFF_HYBRID_MEMORY_TYPE
use ATTR_REL_POS of dimms and mcas as index values.
Need this update to avoid using default 0xFF value
as an index.

Change-Id: I71741bf4e0bd183fe65e1d14b1a53017df7053b6
Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/70056
Tested-by: Jenkins Server <[email protected]>
Tested-by: Jenkins OP Build CI <[email protected]>
Tested-by: FSP CI Jenkins <[email protected]>
Reviewed-by: Daniel M. Crowell <[email protected]>